I play as a siren and every now and then I will play with a psycho friend. So my wonder is, when I phaselock or use scorn I put Kreig into ffyl. I was thinking that it has to do with my skills along with his but not quite sure.
I thought I saw this topic somewhere 'round here but couldn’t find it.
He is most likely speced into fuel the rampage. Which let’s him take friendly fire damage.
What @Piemanlee said. You probably have specced into Maya’s skills that add to the initial phaselock blast damage and, if your friend is too close, they get hit too. Note that this will also allow you to (1) run them over using a vehicle to put them in FFYL and then (2) Resurrect them repeatedly. I do not recommend doing this if you wish to keep them as a friend, however!
Btw if you spec into restoration it will cancel out the skill and let you heal him with bullets.
Thanks guys, figured it was something like that but I’m not very familiar with Kreig skills.
I’m using ruin, scorn and 15 points to have converge so, BOOM! down he goes, sigh.
Yeah, @Piemanlee, I have a point in restore…only 1 because for now he is at a lower level.
@VaultHunter101, good point about him being to close as I wasn’t quite sure and noticed a slight difference depending on where he was.
It’s Fuel the Rampage. If your friend has Release the Beast, tell him to spec out of FtR because he doesn’t need it.
Before getting Release the Beast at LV 31 (earliest), a Psycho still can come along without Fuel the Rampage, if the Siren (You) has enough Team-healing like Sweet Release to keep the Psycho alive while he soaks up the damage to reset his Cooldown. He needs to receive health-damage of around 200% of his max-health, so he will need the healing. It will be a little less of a perma-rampage, but your Psycho-buddy will still enjoy him-/herself.
Oh and pro-tip for NVHM and TVHM: Krieg should use Light the Fuse to stay mobile while fighting for his/her life. Outside of UVHM, its strong enough to more-or-less ensure second winds even in 2-player-mode.
When your friend has speced into Release the Beast, you need to spec out of Sweet release and Elated (if you have them, that is…). Otherwise that healing on your friend may throw off his timing on RtB. Something to keep in mind! 
Be sure to tell him to not use the Love Thumper. When me and a buddy used to play together I loved the shield but he didn’t. It has a large blast radius that harms both friend and foe so unless you stay really far back it can and will knock you and anyone else into FFYL.
Yes, @Ronnie_Rayburn, I use both but can switch it up a bit for when playing with him.
Thanks again folks, very helpful and informative hearing others ideas.
Sweet Release can be worked around because the healing trails are fairly slow. I agree on Elated though.